I recently sold a BN raincover for a Loola & overcharged on postage by £2.50.
I contacted the buyer & explained that I had overcharged & needed to refund but as I had no funds in my PP account could he give me his email address so I could send the funds.
The buyer left positive feedback & just sent me this message 'No need to, thanks though. just call it a thank you payment as its beltin, cheers'